Description: When testing and troubleshooting an electrical circuit, the street rod technician often checks and measures voltage drop on different components or parts. Voltage must be present for any amperage to flow through a resistor or electrical load. As the current flows through a resistor or load, the voltage is dropped across that resistor. Checking how much voltage is lost across a resistor or load is referred to as checking voltage drop. To mathematically determine the voltage drop across any resistor, use Ohm’s Law. In this case, however, use only the amperage and resistance at that particular resistor or load. Voltage drop at any resistor is shown as:Voltage drop = Amperage x Resistance (I x R) at any one resistor.

Voltage drop can also be measured by using a voltmeter. The voltmeter leads are placed across the load or component as shown in the drawing to the left. In this case a 12 volt battery supplies voltage and amperage for a series circuit with two resistors. The voltmeter is checking the voltage drop across the resistor labeled as R 2. The voltage drop of R 1 plus the voltage drop of R 2 will equal the source voltage or 12 volts.

Here are some rules to keep in mind when using a voltmeter.

1. Always ground the negative lead and probe the circuit with the positive lead.

2. Voltmeters are always connected in parallel with the circuit.

3. Use a voltmeter when testing for an open circuit or failed component.

4. Use a digital voltmeter with a 10 megaohm impedance (or higher) rating when testing a circuit that has solid state components. Relationship to Street Rods: The voltmeter can be a valuable tool when testing and troubleshooting an electrical circuit. Think of a simple circuit used for a light in the trunk of a street rod. The circuit has a light and a switch to turn the light off and on. After connecting the wires, it’s found that the light does not turn on when the switch is closed. So the problem is either in the switch, or the light bulb assembly, or their connections. Using a voltmeter, check for voltage drop across the switch when the switch is closed. Normally, the switch is closed so there should be no voltage drop across it. However, after checking, there is a voltage drop across the switch. The voltage should be dropped across the light, not the switch when in the closed position. In this case, either the switch could be bad or the wire connections to the switch could be bad. This shows that checking voltage drop can be a very useful tool to determine the exact problem within an electrical circuit.